Conventionally, a mobile crane is mounted with a winch device driven by a hydraulic actuator such as a hydraulic motor to perform a hoisting work of raising or lowering a hoisted load by the winch device.
Also, it has been proposed to mount an electric winch device using an electric actuator having a greater energy efficiency than the hydraulic actuator on a crane for energy saving. For example, it has been proposed to mount an electric winch device on a tower crane to be fixedly installed on a floor base of a building under construction for a crane operation. Patent Literature 1 shows a climbing crane as an example of the tower crane.
The climbing crane disclosed in Patent Literature 1 includes an electric winch device having a winch drum and two motors to drive the drum. In the electric winch device, the two motors are arranged on the opposite sides of the winch drum in an axial direction of the winch drum in order to prevent occurrence of a twist at a local portion of an input shaft or a body of the winch drum, and torques are input from the respective motors on the opposite sides to the winch drum.
Although the electric winch device of Patent Literature 1 has an advantage that a sufficient hoisting performance can be assured by rotating the winch drum by a torque output from the two motors, it is difficult to mount the electric winch device on a mobile crane in terms of a dimensional restriction for the mobile crane. The reason for the difficulty will be described below.
In a mobile crane, a winch device is mounted on an upper slewing body. The upper slewing body will involve a dimensional limitation when the upper slewing body is transported or the mobile crane runs on a public road.
Specifically, the upper slewing body is placed in a transportation vehicle such as a trailer for transportation of the mobile crane. In this case, normally, the upper slewing body is placed in such manner that the width direction of the upper slewing body is along the width direction of the transportation vehicle. The transportation vehicle runs on a public road, and hence is required to satisfy a statutory regulated vehicle width restriction. Accordingly, the width of the upper slewing body is also limited in accordance with the vehicle width restriction. In the case that the upper slewing body is mounted with the winch device of Patent Literature 1, which is provided with two motors on the opposite sides of the winch drum in the axial direction of the winch drum, in the arrangement that the axial direction of the winch drum is along the width direction of the upper slewing body, the electric winch device has an increased dimension in the axial direction of the winch drum. Therefore, it is difficult to mount the electric winch device on the upper slewing body because of the above-described width limitation of the upper slewing body.
Moreover, when a mobile crane runs on a public road, the mobile crane is required to satisfy the vehicle width restriction. The upper slewing body is arranged in such a manner that the width direction of the upper slewing body is along the vehicle width direction of the mobile crane, when the mobile crane runs on a public road. Hence, the width of the upper slewing body is limited in accordance with the vehicle width restriction in this case as well. Therefore, it is also difficult to mount the electric winch device of Patent Literature 1 on the upper slewing body.